1. Field of the Invention
The present invention relates to an optical amplifier for optical communication, and more particularly to a Raman optical amplifier that includes a Raman pumping light source and a fiber amplifier in order to Raman-amplify an optical signal.
2. Description of the Related Art
For long-distance transmission of optical signals, optical communication networks use various optical amplifiers adapted to amplify the intensity of an optical signal to be transmitted. However, such optical amplifiers not only amplify the intensity of an optical signal to be transmitted, but also amplify noise contained in the optical signal.
The erbium-doped fiber amplifier is mainly used in backbone networks and metro networks by virtue of its high gain, low noise figure, and high saturated output power. However, it is expensive and relatively large in size, as compared to other optical amplifiers, even though it has many advantages as mentioned above. Semiconductor optical amplifiers feature low cost and relatively wide amplification band. However, they have limited application due to their high noise characteristics. To avoid increasing noise upon amplification without encountering the above problems, a Raman optical amplifier has been proposed which is configured by adding a Raman pumping light source to an optical amplifier having a conventional configuration.
FIG. 1 illustrates a Raman optical amplifier including a conventional fiber amplifier and a Raman pumping light source. The Raman optical amplifier module includes a Raman optical fiber amplifier 110, a Raman pumping light source 130 for Raman pumping the fiber amplifier 110, and a semiconductor optical amplifier (SOA) 140 for secondarily amplifying an optical signal Raman-amplified by the fiber amplifier 110. The Raman optical amplifier module also includes a first isolator 150, a second isolator 160, and a wavelength selective coupler (WSC) 120.
The fiber amplifier 110 is Raman-amplified by a Raman pumping light of a predetermined wavelength, thereby Raman-amplifying an optical signal inputted thereto. For the fiber amplifier 110, a single-mode optical fiber may be used.
The first isolator 150 outputs, to the fiber amplifier 110, an optical signal to be amplified. The first isolator 150 also prevents the optical signal, amplified by the fiber amplifier 110, from flowing back, thereby minimizing loss of the optical signal.
The Raman pumping light source 130 serves to output the Raman pumping light of predetermined wavelength for Raman-pumping the fiber amplifier 110. For the Raman pumping light source 130, a semiconductor laser may be used.
For the wavelength selective coupler 120, an arrayed waveguide grating having a Y-branch structure may be used. The wavelength selective coupler 120 is connected to the Raman pumping light source 130, fiber amplifier 110, and semiconductor optical amplifier 140. Accordingly, the wavelength selective coupler 120 outputs the Raman pumping light emitted from the Raman pumping light source 130, while outputting the Raman-amplified optical signal from the fiber amplifier 110 to the semiconductor optical amplifier 140.
The semiconductor optical amplifier 140 performs a secondary amplification for the Raman-amplified optical signal from the fiber amplifier 110, that is, re-amplifies the Raman-amplified optical signal, and then outputs the resultant optical signal to the second isolator 160. As the optical signal already amplified in the fiber amplifier 110 is re-amplified by the semiconductor optical amplifier 140, an improvement in amplification gain is achieved. Also, since the optical signal is Raman-amplified in the fiber amplifier 110, it is possible to reduce the noise caused by the amplification of the optical signal carried out in the semiconductor optical amplifier 140.
The second isolator 160 externally outputs the optical signal amplified by the semiconductor optical amplifier 140. The second isolator 160 also serves to prevent the optical signal, inputted thereto, from flowing into the semiconductor optical amplifier 140.
In a Raman fiber amplifier having the above mentioned configuration, however, it is necessary to additionally use a separate pumping light source for Raman-amplifying the fiber amplifier, and a semiconductor optical amplifier. Energization of the pumping light source and semiconductor optical amplifier increases power consumption. The addition of the pumping light source and semiconductor optical amplifier also increases manufacturing costs and the volume of the Raman fiber amplifier.